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<p>Reduces (<a href="http%3A//en.wikipedia.com/wiki/Fold_%28higher-order_function%29" class="extiw" title="enwiki:Fold (higher-order function)">folds</a>) a <a href="parameter_pack.html" title="cpp/language/parameter pack">parameter pack</a> over a binary operator.
</p>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Syntax">edit</a>]</span> <span class="mw-headline" id="Syntax">Syntax</span>
</h3>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td>
</tr>
<tr class="t-sdsc">
<td> <code><b>(</b></code> <span class="t-spar">pack</span> <span class="t-spar">op</span> <code><b>...</b></code> <code><b>)</b></code>
</td>
<td> (1)
</td>
<td> <span class="t-mark-rev t-since-cxx17">(since C++17)</span>
</td>
</tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td>
</tr>
<tr class="t-sdsc">
<td> <code><b>(</b></code> <code><b>...</b></code> <span class="t-spar">op</span> <span class="t-spar">pack</span> <code><b>)</b></code>
</td>
<td> (2)
</td>
<td> <span class="t-mark-rev t-since-cxx17">(since C++17)</span>
</td>
</tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td>
</tr>
<tr class="t-sdsc">
<td> <code><b>(</b></code> <span class="t-spar">pack</span> <span class="t-spar">op</span> <code><b>...</b></code> <span class="t-spar">op</span> <span class="t-spar">init</span> <code><b>)</b></code>
</td>
<td> (3)
</td>
<td> <span class="t-mark-rev t-since-cxx17">(since C++17)</span>
</td>
</tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td>
</tr>
<tr class="t-sdsc">
<td> <code><b>(</b></code> <span class="t-spar">init</span> <span class="t-spar">op</span> <code><b>...</b></code> <span class="t-spar">op</span> <span class="t-spar">pack</span> <code><b>)</b></code>
</td>
<td> (4)
</td>
<td> <span class="t-mark-rev t-since-cxx17">(since C++17)</span>
</td>
</tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td>
</tr>
</table>
<div class="t-li1">
<span class="t-li">1)</span> unary right fold</div>
<div class="t-li1">
<span class="t-li">2)</span> unary left fold</div>
<div class="t-li1">
<span class="t-li">3)</span> binary right fold</div>
<div class="t-li1">
<span class="t-li">4)</span> binary left fold</div>
<table class="t-par-begin">


<tr class="t-par">
<td>  <span class="t-spar">op</span>
</td>
<td> -
</td>
<td>  any of the following 32 <i>binary</i> operators: <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">+</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">-</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">/</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">%</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">^</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">&amp;</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">|</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">&lt;</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">&gt;</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">&lt;&lt;</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">&gt;&gt;</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">+</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">-</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">/</span><span class="sy1">=</span></span></span>  <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">%</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">^</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">&amp;</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">|</span><span class="sy1">=</span></span></span>  <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">&lt;&lt;=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">&gt;&gt;=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">==</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">!</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">&lt;=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">&gt;=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">&amp;&amp;</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">||</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp">,</span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp">.<span class="sy2">*</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">-</span><span class="sy1">&gt;</span><span class="sy2">*</span></span></span>. In a binary fold, both <span class="t-spar">op</span>s must be the same. <a href="operator_precedence.html" title="cpp/language/operator precedence">Precedence</a> of this operator relative to either <span class="t-spar">pack</span> or <span class="t-spar">init</span> expressions is the highest (equivalent to unary operators)
</td>
</tr>
<tr class="t-par">
<td>  <span class="t-spar">pack</span>
</td>
<td> -
</td>
<td>  any expression that contains an unexpanded <a href="parameter_pack.html" title="cpp/language/parameter pack">parameter pack</a>
</td>
</tr>
<tr class="t-par">
<td>  <span class="t-spar">init</span>
</td>
<td> -
</td>
<td>  any expression that does not contain an unexpanded <a href="parameter_pack.html" title="cpp/language/parameter pack">parameter pack</a>
</td>
</tr>
</table>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span>
</h3>
<p>The instantiation of a <i>fold expression</i> expands the expression <code>e</code> as follows:
</p>
<div class="t-li1">
<span class="t-li">1)</span> Unary right fold <span class="texhtml" style="white-space: nowrap;">(E op ...)</span> becomes <span class="texhtml" style="white-space: nowrap;">E<span class="t-su t-su-b"><br>1</span> op (... op (E<span class="t-su"><br></span> op E<span class="t-su t-su-b"><br>N</span>))</span>
</div>
<div class="t-li1">
<span class="t-li">2)</span> Unary left fold <span class="texhtml" style="white-space: nowrap;">(... op E)</span> becomes <span class="texhtml" style="white-space: nowrap;">((E<span class="t-su t-su-b"><br>1</span> op E<span class="t-su t-su-b"><br>2</span>) op ...) op E<span class="t-su t-su-b"><br>N</span></span>
</div>
<div class="t-li1">
<span class="t-li">3)</span> Binary right fold <span class="texhtml" style="white-space: nowrap;">(E op ... op I)</span> becomes <span class="texhtml" style="white-space: nowrap;">E<span class="t-su t-su-b"><br>1</span> op (... op (E<span class="t-su t-su-b"><br>N−1</span> op (E<span class="t-su t-su-b"><br>N</span> op I)))</span>
</div>
<div class="t-li1">
<span class="t-li">4)</span> Binary left fold <span class="texhtml" style="white-space: nowrap;">(I op ... op E)</span> becomes <span class="texhtml" style="white-space: nowrap;">(((I op E<span class="t-su t-su-b"><br>1</span>) op E<span class="t-su t-su-b"><br>2</span>) op ...) op E<span class="t-su t-su-b"><br>N</span></span>
</div>
<p>(where <span class="texhtml" style="white-space: nowrap;">N</span> is the number of elements in the pack expansion)
</p>
<p>For example, 
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span>... <span class="me1">Args</span><span class="sy1">&gt;</span>
<span class="kw4">bool</span> all<span class="br0">(</span>Args... <span class="me1">args</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="br0">(</span>... <span class="sy3">&amp;&amp;</span> args<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
 
<span class="kw4">bool</span> b <span class="sy1">=</span> all<span class="br0">(</span><span class="kw2">true</span>, <span class="kw2">true</span>, <span class="kw2">true</span>, <span class="kw2">false</span><span class="br0">)</span><span class="sy4">;</span>
 <span class="co1">// within all(), the unary left fold expands as</span>
 <span class="co1">//  return ((true &amp;&amp; true) &amp;&amp; true) &amp;&amp; false;</span>
 <span class="co1">// b is false</span></pre></div></div>
<p>When a unary fold is used with a pack expansion of length zero, only the following operators are allowed:
</p>
<div class="t-li1">
<span class="t-li">1)</span> Multiplication (<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span></span></span>). The value for the empty pack is <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">1</span></span></span>
</div>
<div class="t-li1">
<span class="t-li">2)</span> Addition (<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">+</span></span></span>). The value for the empty pack is <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span><span class="br0">(</span><span class="br0">)</span></span></span> (zero)</div>
<div class="t-li1">
<span class="t-li">3)</span> Bitwise AND (<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">&amp;</span></span></span>). The value for the empty pack is <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">-</span><span class="nu0">1</span></span></span> (bitwise all ones)</div>
<div class="t-li1">
<span class="t-li">4)</span> Bitwise OR (<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">|</span></span></span>). The value for the empty pack is <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span><span class="br0">(</span><span class="br0">)</span></span></span> (zero)</div>
<div class="t-li1">
<span class="t-li">5)</span> Logical AND (<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">&amp;&amp;</span></span></span>). The value for the empty pack is <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span>
</div>
<div class="t-li1">
<span class="t-li">6)</span> Logical OR (<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">||</span></span></span>). The value for the empty pack is <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span>
</div>
<div class="t-li1">
<span class="t-li">7)</span> The comma operator (<span class="t-c"><span class="mw-geshi cpp source-cpp">,</span></span>). The value for the empty pack is <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span><span class="br0">(</span><span class="br0">)</span></span></span>
</div>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Example">edit</a>]</span> <span class="mw-headline" id="Example">Example</span>
</h3>
<div class="t-example">
<div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> ...<span class="me1">Args</span><span class="sy1">&gt;</span>
<span class="kw4">void</span> printer<span class="br0">(</span>Args<span class="sy3">&amp;&amp;</span>... <span class="me1">args</span><span class="br0">)</span> <span class="br0">{</span>
    <span class="br0">(</span><a href="../io/cout.html"><span class="kw1446">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> ... <span class="sy1">&lt;&lt;</span> args<span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    printer<span class="br0">(</span><span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">3</span>, <span class="st0">"abc"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">123abc</pre></div></div> 
</div>

<!-- 
NewPP limit report
Preprocessor visited node count: 2339/1000000
Preprocessor generated node count: 5723/1000000
Post‐expand include size: 41971/2097152 bytes
Template argument size: 9330/2097152 bytes
Highest expansion depth: 15/40
Expensive parser function count: 0/100
-->

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